Arrangements for magnetic amplifier circuit selection



July 8, 1958 .w. s. KINSEY ETAL 2,342,734

. ARRANGEMENTS FOR MAGNETIC AMPLIFIER CIRCUIT SELECTION Filed Nov. 5, 1956 Unite Patent ARRANGEMENTS FDR MAGNETIC AMPLIFIER CIRCUIT SELECTION Ronald William Stanley Kinsey, West Dulwich, London, and Brian Desmond fiimmons, Greenwich, London, England, assignors, by means assignments, to Siemens Edison Swan Limited, London, England, a British com- P y Application November 5, 1956, Serial No. 620,503

Claims priority, application Great Britain Ntwenuber 4, 1955 2 Claims, (Cl. 323-89) This invention relates to improved circuit arrangements for the selection of a free one of a number of circuits in a group, and particularly to such circuit arrangements (hereinafter termed electronic selecting circuits of the kind referred to) which include for the purpose of selection devices, such as cold cathode gas discharge tubes or transistors, having a negative resistance over part of their characteristics.

Electronic selecting circuits of the kind referred to are well known, and frequently take the form of a group of three-electrode cold cathode gas discharge tubes which have their anode or cathode electrodes connected in parallel, the common anode-or cathode connection being connected to the appropriate terminal of a source of supply voltage over a single anode or cathode impedance. In the case of an electronic selecting circuit of this form, when a stimulus is applied simultaneously to the striker electrodes of all the tubes only one ofthe tubes is able to establish a discharge between its anode and cathode electrode. This is due to the fact that in establishing such a discharge a finite time is required for any tube to pass through the negative resistance region of its characteristic, and since the rate at which this region of a tube characteristic is traversed depends on a number of factors inherent in the tube itself, it rarely occurs that two tubes will traverse their negative resistance regions in the same time. Thus, in practice, it is found that one tube of a group is able to traverse its negative resistance region and establish a discharge between its anode and cathode electrodes'before any of the others. The passage of this discharge current through the common anode or cathode impedance results in the potential difference across the tubes'of the group being reduced below the striking value with the result that a lock out of the remaining tubes occurs. Often the common anode or cathode impedance is arranged to include a certain amount of inductance which serves to prolong the times of traverse of the tubes over the-negative resistance. regions .of their characteristics and thus serves to improve the reliability of the so called one only selection effected by the circuit. v, While thesimple form of electronic selecting circuit of the kind referred to which has just been described is satisfactory in many respects, a ditficulty arises in practice when it is required to obtain a signal from the tube through which a discharge has been achieved, since the introduction in the individual discharge path of each of the tubes of a positive resistance of a value such that a suitable signal voltage may be developed across it tends to cancel the efiect of the negative resistances of the tubes and so cause the circuit to become unreliable in regard to its one only. selecting function.

The present invention has as its main object the provision of improved electronic selecting circuits of-the kind referred to in which the difiic'ulty that arises in regard to obtaining a signal from an individual path having 2,842,734 Patented July 8, 1958 'ice a negative resistance region in its characteristic, and traversed by current indicative of the selection of the circuit to which the path pertains, is so materially minimised that it is for practical purposes overcome.

The present invention is based on the realisation of the fact that if a magnetic amplifier is used to amplify the change of current, through a cold cathode gas discharge tube or other equivalent individual path, which is indicative of the selection of the circuit to which this particular individual path pertains, then the direct current magnetisiug winding (of the magnetic amplifier used) which is connected in the individual path can be designed to have a positive resistance which is sufiiciently low to have no material effect on the reliability of the selecting circuit in regard to its one only selecting function, while still permitting an adequate output signal to be obtained from an alternating current winding of the magnetic amplifier.

According to a main feature of the invention, in an electronic selecting circuit of the kind referred to there is provided, individual to each of the plurality of similar discharge or other negative resistance paths that serve for effecting selection, an iron cored saturable inductor having a low resistance direct current magnetising winding connected in, and arranged to be energized by the current in, the particular path, and having an alternating current winding which is connected tocontrol the current flowing, in a circuit individual to the'particular path, between a source of alternating current and an output terminal corresponding to the particular path.

With an electronic selecting circuit according to this feature in which cold cathode discharge tubes constitute the negative resistance paths that serve for effecting selection, in the absence of any discharge current through a particular cold cathode discharge tube the associated saturable inductor allows only a negligible current to pass through its alternating current winding, but when the said discharge tube does pass a substantial discharge current the saturable inductor allows a considerable current to pass through its alternating current winding. This alternating current may be rectified if desired and a direct current signal developed.

According to another important feature of the invention, an electronic selecting circuit of the kind referred to includes a cold cathode gas discharge tube having a plurality of discharge gaps formed between a like plurality of cathode electrodes and a common anode electrode,'the tube being so arranged, and so connected with impedance in the circuit of said anode electrode and as .for-its completion or effectiveness upon the closure of a normally open switching device individual to the particular cathode'electrode, whereby, upon the simultaneous closure of a plurality of the switching devices arising from a demand for a free circuit in said group, a

discharge is established from said common anode electrode to one and only one of the plurality of said cathode electrodes associated with the closed switching devices and an output signal is produced at an output terminal associated with an alternating current winding ,of the particular output magnetic amplifier corresponding to the cathode electrode to which a discharge has been established. The term switching device is to be taken as including within its meaning both so-called static and dynamic switching devices. For example, a

suitable switching device may comprise a cold cathode discharge tube arranged to be triggered in response to a suitable potential applied to a striker electrode, or it may comprise a switching magnetic amplifier (additional to the output magnetic amplifier connected in circuit upon the closure of the device), or it may comprise a contact of an electromagnetic relay.

We have found that by using a cold cathode gas discharge tube of the rnulti-cathode type in the way just set forth, a completely reliable one only selection can be achieved without the need for any inductance in the anode impedance, and that the speed with which the one only selection is effected is materially faster than the corresponding speed obtained in the case of previously-known electronic selecting circuits of the kind referred to in which individual cold cathode gas discharge tubes are employed and inductance is included in a common anode or cathode impedance to increase the reliability of selection.

A better understanding of the nature of the invention will be gained from the following description, with reference to the accompanying drawing, of two exemplary embodiments of the invention. Fig. 1 of the drawing is a circuit diagram illustrating the particular form of electronic selecting circuit of the kind referred to which constitutes one of these two exemplary embodiments, and Pig. 2 is a circuit diagram illustrating the particular form of electronic selecting circuit of the kind referred to which constitutes the other of these two exemplary embodiments.

In the exemplary embodiment to which Fig. 1 pertains, there is a cold cathode discharge tube lDl of a multi-cathode type comprising a number of main cathode electrodes 11 arranged about a common anode electrode 12. In the circuit external to the tube 1D1, the anode electrode 12 is connected over an anode load resistor 1R1, having a resistance of the order of 390,000 ohms, to a source of positive potential of about 375 volts. An auxiliary cathode electrode 13 of the tube 1131 is connected to earth over a current limiting resistor 1R2 of about 20 megohms resistance, this connection serving to maintain an ionising discharge in the tube. Each utilised main cathode electrode 11 of the tube lDf. is connected over the direct current control winding (I) of an output magnetic amplifier such as 151 to the anode of a three-electrode cold cathode discharge tube such as iCTl, there being one tube such as 1CT1 and one output magnetic amplifier such as 181 individual to each utilised main cathode electrode 11. For the sake of simplicity, only the connections to the extreme lefthand one (as shown) of the main cathode electrodes ill of the tube lDll are shown in the figure. The connections to each of the remaining main cathode electrodes correspond exactly to those just mentioned. The direct current control winding (1) of each magnetic amplifier such as 151 has a rectifier such as lMRl connected across it. This rectifier serves to provide positive feedback, by producing even-harmonic selfexcitation of the amplifier, in known manner.

The alternating current winding of each output magnetic amplifier comprises two sections (II) and (III) connected in series between a source lACS of alternating current having a frequency of the order of 6000 cy ies per second and an output terminal such as IT which in turn is connected to earth over a load resistor such as across which the output signal is developed, this lead resistor having a resistance of the order of 10,000 ohms. Each three-electrode tube such as ICTI also has its anode connected to a source of positive potential of approximately 150 volts over an anode load resistor such as 1R4 of about 180,000 ohms resistance individual to the tube. Each such anode load' resistor is shunted by a rectifier such as IMRZ which is connected in the sense in which it permits current flow towards the said 150 volt source, and which serves to prevent the anode potential of the relevant three-electrode tube rising above volts at the instant of application of the 375 volt high tension supply to the anode electrode 12 of the multi-cathode discharge tube lDl. The cathodes of each of the three-electrode discharge tubes such as ICU. are directly connected to a source of negative potential of about 50 volts.

When the multi-cathode discharge tube lDl is connected in circuit ready to perform its functions for the selection of a free one of a number of circuits in a group there being a main cathode electrode 11 fully connected as described and illustrated for, and corresponding to, each of these circuits), a glow discharge of low intensity is established between the anode electrode 12 and the earth-connected auxiliary cathode electrode 33. When a free one of the said group of cir cuits is to be selected, the free circuits of the group apply stimuli (striking potentials) simultaneously to the striker electrodes 14 of their respective associated threeelectrode cold cathode discharge tubes such as lCTl, and these tubes strike and draw a discharge current through their respective associated anode load resistors such as 1R4, causing a drop in potential of the respective anodes and also, therefore, of the corresponding main cathode electrodes 11 of the rnulti-cathode discharge tube EDI, thereby effectively marking these cathode electrodes for selection. This drop in potential is such as to reduce the potential of the marked main cathode electrodes 11 to a value lower than that of the earthconnected auxiliary cathode electrode 113, consequently the ionising glow discharge previously established between the anode electrode 12 and auxiliary cathode electrode 13 is caused to transfer to one of the marked main cathode electrodes 11. Upon transfer, the intensity of the discharge through the multi-cathode tube IDI is substantially increased and this results in the voltage drop across the anode resistor 1R1 of this tube increasing to such a value as to ensure a lock out of the remaining cathode electrodes. With this arrangement no appreciable current flows in the direct current control winding (1) of any of the output magnetic amplifiers such as 181 until transfer of the glow within the multi-cathode discharge tube 1D]. from the auxiliary cathode electrode 13 to one of the main cathode electrodes lll marked by a discharging three-electrode tube such as llCTl occurs. Consequently, prior to such transfer, only a small, and ineffective, current flows in the alternating current windings of the output magnetic amplifiers, but upon such transfer, the direct current which flows through the control winding (I) of the output magnetic amplifier associated with the particular main cathode electrode 11 to which transfer of the glow has occurred results in a substantial flow of current in the alternating current winding, comprising winding sections (1) and (1H), of this magnetic amplifier, with the result that an effective output signal is developed at the relevant output terminal such as IT.

Preferably, the saturable inductor employed to form each output magnetic amplifier such as 131 is one having a three-limbed core, section (II) of the alternating current winding being wound on one of the two outer limbs, section (III) of the alternating current winding being wound on the other of the two outer limbs, the direct current control winding (I) being wound on the central limb, and it being arranged that, so far as the fundamental alternating current frequency of about 6000 cycles per second is concerned, the two sections (II) and (III) of the alternating current winding assist in producing flux around the two outer limbs and neutralise each other in regard to the production of flux in the central limb. With such a saturable inductor, when the output magnetic amplifier it forms is in the quiescent condition, i. e. the condition which Obtains when the multi-cathode discharge tube 1B1 is not producing current flow through the direct current control winding (I), no appreciable alternating voltage'of any frequency is induced'in the control winding (1), and the connection over the rectifier such as IMRI is'without eifect. When substantial control current flows through the control winding (1) upon the transfer of the glow within the multi-cathode discharge tube 1D1'to the relevant main cathode electrode 11, the resulting operation ofthe saturable inductor over a' non lin'ear portion of its mag-' netisationcharacteristic gives rise to, the production of alternating voltages of even harmonic frequencies in the control winding (1), the alternating voltage of second harmonic frequency being of material magnitude; In

these circumstances, the connection over the rectifier such as 1MR1 becomes effective, the rectifier serving to produce, 'from'the alternating voltages induced in the control winding (I), a unidirectional current throughthis winding which assists the control current from the multicathode discharge tube 1D1 and thereby in known manner, provides positive feedback. 7

In using the electronic selecting circuit of Fig. 1, provision has of course to be made for restoring the circuit to normal after use; Such restoration may be effected by changing the potential applied to the cathodes of all the three-electrode discharge tubes such as 1CT1 from a negative potential of about 50 volts to a positive one of about 150 volts. "It should be. noted that the mere removal of the applied stimuli (striking' potentials) from the striker electrodes 14 of the discharging ones of the three-electrode discharge tubes such as lCTl will not in itself suflice to bring about such restoration.

Referring now to Fig. 2, the exemplary embodiment to which this figure pertainsis similarcin many respects to the "embodiment illustrated in-Fig.i1, theonly important difference being that the marking, for selection, of the main cathode electrodes of the multi-oathode discharge tube employed is controlled by switching devices constituted by switching magnetic amplifiers instead of being controlled by three-electrode cold cathode discharge tubes. The elements ZACS, 2D1, 2MR1, 2R1, 2R2, 2R3, 281, and 2T correspond to the elements 1ACS, lDl, 1MR1, 1R1, 1R2, 1R3, 181, and 1T respectively of Fig. 1. The anode electrode of the multi-cathode discharge tube 2D1 is connected over the anode load resistor 2R1 to a source of positive potential of about 400 volts. Each utilised main cathode electrode of the tube 2D1 is connected over the direct current control winding (I) of the output magnetic amplifier such as 231, and a rectifier load resistor such as 2R4 having a resistance of about 820,000 ohms, to a source of positive potential of about 230 volts, there being of course a rectifier load resistor such as 2R4 individual to each such electrode. Each utilised main cathode electrode of the tube ZDI also has individual to it a switching magnetic amplifier such as 282, a coupling resistor such as 2R5, and a voltage-doubler rectifier circuit comprising elements such as two capacitors 2C1 and 2C2 and two rectifiers such as 2MR2 and 2MR3, the relevant rectifier load resistor such as 2R4 being connected to act as a load resistor for this voltage-doubler rectifier circuit.

The alternating current winding of each switching magnetic amplifiers such as 282 comprises two sections (II) and (III) connected, in series with the coupling resistor such as 2R5, between a source ZAS of alternating current and earth. The source 2A8 may conveniently be the same as the source ZACS (having a frequency of the order of 6000 cycles per second), each coupling resistor such as 2R5 then having a resistance of about 10,000 ohms, and the capacitors such as 2C1 and 2C2 then each having a capacity of the order of 0.002 microfarad. The saturable inductor employed to form each switching magnetic amplifier such as 282 may be of the same general type as the saturable inductors employed to form the output magnetic amplifiers such 2S1. In the absence of control current in the direct current control winding (I) of a switching tifier circuit produces no material output.

magnetic amplifier such as 252, only a small, and ineffective, current flows in the alternating current winding of the amplifier, and the corresponding voltage-doubler rec- No provision is made for positive feedback in the case of the switching magnetic amplifiers such as 282.

When the multi-cathode discharge tube 2D1 is connected in circuit ready to perform its functions for the selection of a free one of a number of circuits in a group, a glow discharge oflow intensity is established between the anode electrode and the earth-connected auxiliary cathode electrode of the tube, just as in the case of the tube 1D1 of Fig. 1. As will be apparent from the description already given with reference to Fig. 1, there is a main cathode electrode fully connected as described (and as illustrated in Fig. 2 for one such electrode) for and corresponding to each circuit in the group concerned. When a free one of the circuits in the group is to be selected, the free circuits of the group produce efiective control currents simultaneously in the direct current control windings (I) of their associated switching magnetic amplifiers such as 282, with the result that a greatly increased alternating current flows in the alternating current winding of each switching magnetic amplifier affected. In the case of each switching magnetic amplifier affected, the increased alternating current in its alternating current winding produces, across the, relevant coupling resistor such as.2R5, an alternating voltage sufficient to cause the associated voltage-doubler rectifier circuit to produce,

across the relevant rectifier load resistor such as 2R4, a voltagewhich, acting in opposition to the positive potentialof about 230 volts applied to one end of this rectifier load resistor, sufiices to reduce the potential of the particular main cathode electrode (of the tube 2D1) to which this rectifier load resistor is connected to a value lower than that of the earth-connected auxiliary cathode of the tube ZDl, thereby effectively marking the main cathode electrode concerned for selection. The further operation of the electronic selecting circuit in effecting a one only selection will be apparent from the description already given with reference to Fig. 1. It should be noted that the voltage-doubler rectifier circuit associated with a main cathode electrode of the tube 2D1 provides, so far as current flowing from the electrode is concerned, a low-resistance shunt across the rectifier load resistor such as 2R4 associated with the electrode.

The electronic selecting circuit of Fig. 2 has the advantage that the mere cutting off of the control currents in the direct current control windings (I) of all the switching magnetic amplifiers such as 282 sufiices to restore the circuit to normal after use.

What we claim is:

1. An electronic selecting circuit, providing for the selection of a free one of a number of circuits in a group, comprising a multi-cathode cold cathode gas discharge tube having a plurality of discharge gaps formed between a like plurality of cathodes and a common anode, an impedance connected in the circuit of said anode for ensuring that a discharge can be maintained across only one of said discharge gaps at any time, for at least one of said cathodes a circuit serving to connect the cathode to a source of ionising potential and for each of the remainder of said cathodes an individual cathode circuit serving for the purpose of selection, said remainder comprising a cathode for each circuit in said group, an output terminal for an individual to each of said remainder of said cathodes, an output magnetic amplifier for and individual to each of said remainder of said cathodes, said magnetic amplifier having a direct current magnetising winding included in the individual circuit of the cathode and having an alternating current winding connected in circuit With the particular said output terminal which pertains to the cathode and said magnetic amplifier serving to produce an out-put signal at the last-mentioned output terminal consequent on the establishment of a discharge across the particular one of said discharge gaps that includes the cathode, and a switching device for and individual to each of said remainder of said cathodes for rendering the cathode and its individual circuit eifective for the purpose of selection consequent on a demand for a free circuit in said group arising while the particular circuit in said group to which the cathode pertains is free, said switching device comprising a cold cathode gas discharge tube having an anode and a cathode and a striker electrode and having the discharge gap formed between its anode and cathode connected in the individual circuit of the corresponding cathode of said multi-cathode cold cathode gas discharge tube and serving to lower the potential of the last-mentioned cathode on being struck consequent on the application of a striking potential to said striker electrode.

2. An electronic selecting circuit, providing for the selection of a free one of a number of circuits in a group, comprising a multi-cathode cold cathode gas discharge tube having a plurality of discharge gaps formed between a like plurality of cathodes and a common anode, an impedance connected in the circuit of said anode for ensuring that a discharge can be maintained across only one of said discharge gaps at any time, for at least one of said cathodes a circuit serving to connect the cathode to a source of ionising potential and for each of the remainder of said cathodes an individual cathode circuit serving for the purpose of selection, said remainder comprising a cathode for each circuit in said group, an output terminal for and individual to each of said remainder of said cathodes, an output magnetic amplifier for and individual to each of said remainder of said cathodes, said magnetic amplifier having a direct current magnetising winding included in the individual circuit of the cathode and having an alternating current winding connected in circuit with the particular said out-put terminal which pertains-to the cathode and said magnetic amplifier serving to produce an output signal at the last-mentioned output terminal consequent on the establishment of a discharge across the particular one of said discharge gaps that includes the cathode, and a switching device for and individ ml to each of said remainder of said cathodes for rendering the cathode and its individual circuit effective for the purpose of selection consequent on a demand for a free circuit in said group arising while the particular circuit in said group. to which the cathode pertains is free, said switching device comprising rectifying means having an individual alternating current input circuit and having an output circuit connected in the individual circuit of the corresponding cathode of said multi-cathode cold cathode gas discharge tube and serving to lower significantly the potential of the last-mentioned cathode on receiving an effective alternating current input, and said switching device further comprising a switching magnetic amplifier having an alternating current winding connected to control the current fed to said individual alternating current input circuit of said rectifying means from an alternating current supply and having a direct current magnetising winding and serving to produce an effective alternating current input to said rectifying means consequent on the passage of a control current through said direct current magnetising winding.

References Cited in the file of this patent UNITED STATES PATENTS 

